Miniaturization of semiconductor integrated circuit devices has made difficult extraction of the contour of a mask pattern formed on a photo mask. This has led to the need to extract the contour of the mask pattern by a scanning electron microscope (SEM) that uses charged particle beams.
In images obtained by SEM, a band-like area with a high secondary electron intensity or reflection electron intensity is observed at an edge of a pattern. The band-like area is commonly referred to as a white band.
A peak method and a threshold method are known as representative methods for extracting the contour of a mask pattern. The peak method detects the position of a peak intensity in the intensity distribution of a white band. The threshold method detects the position of an intermediate intensity in the intensity distribution of a white band. In general, the threshold method is excellent in short-term reproducibility. The peak method is excellent in long-term stability.
The reason for the long-term stability of the peak method is as follows. Even with a variation in the width of a white band resulting from various error factors during acquisition of images (for example, a focus error, changes in a primary electron optical system, and vibration of electron beams caused by disturbance), the peak position in the intensity distribution of the white band remains almost unchanged. Thus, the peak method is often used for the contour extraction using SEM.
However, miniaturization of patterns has made the application of the peak method difficult. That is, miniaturized patterns require the addition of fine auxiliary patterns for OPC (Optical Proximity Correction). For example, auxiliary patterns called SRAFs (Sub-Resolution Assist Features) are fine patterns that are not transferred onto a wafer. In such fine patterns, white bands at adjacent edges overlap, and the adjacent white bands fail to be separated from each other. This makes the application of the peak method significantly difficult. On the other hand, the application of the threshold method is also difficult due to its inadequate long-term stability.
Thus, a method has been demanded which enables the contour of a pattern on a photo mask to be appropriately extracted.